Water cooling systems, particularly water cooling towers, require continuous maintenance in order to avoid fouling caused by sludge, scale and microorganisims, and corrosion. These problems are treated by adding various chemicals to the water cooling towers.
One class of compounds useful as dispersants, antiscaling agents and corrosion inhibitors are organophosphorous compounds, particularly organophosphorous carboxylic acids and the phosphonates. These organic phosphorus compounds have the ability to complex metal ions by sequestration or threshold treatment. They also form inhibitive films along metal surfaces. This prevents formation of the crystalline structure in scale. Phosphonates are a well defined class of threshold inhibitors used to prevent scale formation. Typically used phosphonates include aminomethylenephosphonic acid and 1-hydroxyl ethylidene-1,1-diphosphonic acid. A commonly employed organophosphorous carboxylic acid (phosphenite) is 2-phosphonobutane- 1,2,4-tricarboxylic acid which can be purchased from Bayer AG.
Alkali and alkaline earth metal molybdate salts are superior corrosion inhibitors to chromate and nitrite salts used at higher concentrations. Sodium molybdate is an anodic inhibitor and is the most commonly used molybdate inhibitor.
Also used for corrosion inhibition are the aromatic azoles. These are specific corrosion inhibitors for copper and its alloys. They are chemisorbed on the metal surfaces and are capable of excellent protection at a minimal dosage of 1 to 2 parts per million. Generally included within the aromatic azoles are benzotriazole, tolyltriazole, and mercaptobenzothiazole.
Organophosphorous carboxylic acids, phosphonates and azoles are used in combination to provide a corrosion and scale inhibition program. Functionally these are all anionic in solution and therefore generally quite compatible.
In addition to corrosion and scale inhibitors and dispersants, water cooling towers are generally treated with certain biocides or biostats to prevent or inhibit algae, fungi and bacteria. Two general types of these are oxidizing and nonoxidizing. Typical oxidizing biocides are ClO.sub.2, chlorine and chlorine release compounds such as chlorinated isocyanurates, hypochlorites and chlorinated hydantoins. Quaternary ammonium compounds are the primary nonoxidizing biocides and biostats. The quaternary ammonium salts are cationic surface active chemicals which are most effective against algae and bacteria in alkaline pH ranges.
One particular class of quaternary ammonium complexes are polymeric quaternary ammonium salts. These complexes are generally nontoxic and accordingly safer to use. They are formed by reacting a bis tertiary amine with a dihalo compound or a hydroxy halocompound. Several of these compounds are currently used to treat water cooling towers. One such product is WSCP from Buckman Laboratories which is poly[oxyethylene(dimethylimminio)ethylene(dimethylimminio)ethylenedichlori de]. This product is used as a swimming pool algicide at two parts per million(ppm) as a maintenance dose and 5 to 8ppm to rid pools of heavy objectionable algae growth. Swimming pools are more easily treated than water cooling towers. The chlorination of swimming pools and other biocidal treatment prevents acclimatization. Further, swimming pools are not actively aerated. Water cooling towers inherently aerate the water which promotes growth of aerobic microorganisms. The manufacturers of WCSP recommend use of WCSP at 20 to 40 parts per million of active product as a biocide for water cooling towers.
Another polymeric quaternary ammonium compound is Mirapol A-15. This product is poly[N-[3(dimethylammonio)propyl]-N'-[3-(ethyleneoxyethylenedimethylammoni o)propyl] urea dichloride]. This is inhibitory against certain bacteria at 100 parts per million. A third polymeric quaternary ammonium compound is ETC-PQ from Onyx Chemical Company which is .alpha.-4-[1 tris(2-hydroxy ethyl)ammonium chloride-2-butenyl]poly[1 dimethylammonium chloride-2-butenyl]-.omega.-tris(2 hydroxyethyl)ammonium chloride. This also acts as a bacteriostat.
Quaternary ammonium salts are excellent biocides for use in water cooling towers. However because quaternary ammonium compounds are cationic in nature and when combined with corrosion inhibitors, dispersants and antiscalants which are anionic in nature, the compounds tend to salt out via chemical reaction thereby destroying functionality. The corrosion inhibitors and antiscalants are used continuously and remain effective even though used continuously. Biocides and biostats on the other hand are not used continuously. Bacteria and other microorganisms can acclimatize to a particular biocide rendering the biocide totally ineffective. This is particularly a problem when low levels of a biocide are used as is required for continuous application. Of course at extremely high concentration, certain biocides would be effective on a continuous basis. But this would be environmentally objectionable and economically unfeasible. Other biocides and biostats are totally ineffective at low levels used in continuous treatment. For these reasons biocides and biostats are generally applied periodically in high concentration, slug doses which kill a high proportion of bacteria or algae. A typical treatment for a water cooling tower will use multiple slug doses of different biocides to prevent acclimatization to any particular biocide.